Glucocorticoid-inducible TNF receptor (GITR) is a T cell costimulatory type I cell surface transmembrane glycoprotein expressed by activated effector T (Teff) cells and at low levels on B cells, eosinophils, basophils, and macrophages (Nocentini et al., Adv Exp Med Biol 2009; 647:156-73). GITR is constitutively expressed on regulatory T (Treg) cells. Signaling through GITR potentiates T cell receptor activation of T cells and renders Teff cells resistant to Treg-mediated suppression (Stephens et al., J Immunol 2004; 173:5008-20). Determining GITR positivity in tumor tissue may provide an opportunity to target patients that are most likely to benefit from (i.e., be responsive to) anti-GITR immunotherapy, as well as monitoring the status of GITR-positive tumors and efficacy of anti-GITR immunotherapy.